The problem under investigation in this research grant proposal is the central neural regulation of body fluids by the hypothalamo-neurohypophysial neurosecretory system in the goldfish, cat and rat. Using both electrophysiological and morphological techniques at the single-cell level, we will take two complementary approaches to the problem. First, by a combination of antidromic identification, intracellular recording and stimulation of various central neural and systemic sites, we will analyze statistically the spontaneous and post-stimulation firing patterns, the electrical membrane potentials and excitatory and inhibitory synaptic potentials (EPSPs and IPSPs) in magnocellular neuroendocrine cells (MgC) in the goldfish, cat and rat. Then, by our second double-labeled morphological approach, we shall iontophorese fluorescent dyes (Lucifer Yellow or Procion Yellow) into these physiologically studied MgC, obtain three dimensional cell reconstructions and immunocytochemically identify the peptidergic MgC cell type. By correlating the results of these physiological and morphological studies, we will be able to analyze the functional and morphological characteristics of immunoreactive vasopressin (VP)-, oxytocin (OT)-, and enkephalin (ENK)-containing MgC in the mammal and immunoreactive vasotocin (VT)-, isotocin (IT)- and enkephalin (ENK)-containing MgC in the teleost. By studying electrophysiological responses in immunocytochemically defined MgC we should learn whether an input pathway is relevant to the secretion of that particular peptidergic MgC and whether MgC can interact with each other. The rationale for these multiple approaches is that they should allow us to provide direct answers to the two questions most important to the neuroendocrine regulation of the hypothalamo-neurohypophysial system at the level of the single cell: (a) What are the functional and morphological aspects of the immunocytochemically identified peptidergic MgC? (b) What is the role of ENK-MgC in hypothalamo-neurohypophysial fluid regulation?